Fluid loss reducer for high temperature high pressure water based-mud application

ABSTRACT

The present invention concerns a water-based drilling mud for utilization in the drilling of oil wells comprising an aqueous phase wherein the aqueous phase contains an oil soluble polymer in the form of a gel as a fluid loss reducer. The subject invention further reveals a method of lubricating a drilling bit during the drilling of a well which comprises circulating a water-based drilling mud in the vicinity of the drilling bit wherein the water-based drilling mud is comprised of an aqueous phase, and wherein the aqueous phase contains an oil soluble polymer in the form of a gel as a fluid loss reducer.

This is a divisional of U.S. patent application Ser. No. 10/738,456,filed on Dec. 17, 2003 now U.S. Pat. No. 7,101,829, in Group Art Unit1712.

BACKGROUND OF THE INVENTION

Drilling fluids, commonly referred to as drilling muds, are complexmixtures of chemicals. They are required to cool and lubricate the drillbit, suspend formation cuttings, lift them to the surface, and controlformation pressure during the drilling of oil wells.

For the most part, the liquid bases are aqueous solutions, oils oremulsions of aqueous and oily materials, to which viscosifiers,dispersants, emulsifiers, weighting agents, fluid loss agents, pHcontrol agents, salts, lubricants, select polymers, corrosioninhibitors, biocides, are usually added to enable the muds to meet theneeds of particular drilling operations.

Drilling muds are usually classified as either water-based muds (WBM) oroil-based muds (OBM), depending upon the character of the continuousphase of the mud, although water-based muds may contain oil andoil-based muds may contain water. Oil-based muds generally usehydrocarbon oil as the main liquid component with other materials suchas clays or colloidal asphalts added to provide the desired viscositytogether with emulsifiers, polymers and other additives includingweighting agents. Water may also be present, but in an amount notusually greater than 50 volume percent of the entire composition. Ifmore than about 5% volume water is present, the mud is often referred toas an invert emulsion, i.e., water-in-oil emulsion.

Water based muds conventionally comprise viscosifiers, fluid losscontrol agents, weighting agents and other additives includinglubricants, emulsifiers, corrosion inhibitors, salts and pH controlagents. The water makes up the continuous phase of the mud and isusually present in any amount of at least 50 volume percent of theentire composition. Oil is also usually present in minor amounts butwill typically not exceed the amount of the water so that the mud willretain its character as a water continuous phase material.

Potassium-muds are the most widely accepted water mud system fordrilling water-sensitive shales. K⁺ ions attach to clay surfaces andlend stability to shale exposed to drilling fluids by the bit. The ionsalso help hold the cuttings together, minimizing dispersion into finerparticles. Potassium chloride (KCl) is the most widely used potassiumsource. Others are potassium acetate, potassium carbonate, potassiumlignite, potassium hydroxide and potassium salt of partial hydrolyzedpolyacrylamide (PHPA). For rheology control, different types of polymersare used, for example XC polymer (Xanthan gum) and PHPA. For fluid-losscontrol, mixtures of starch and polyanionic cellulose are often used.Carboxymethyl starch (CM starch), hydroxy propyl starch (HP starch),carboxymethylcellulose and sodium polyacrylate (SPA) are also used. PHPAis widely used for shale encapsulation.

Salt-water muds contain varying amounts of dissolved sodium chloride(NaCl) as a major component. Undissolved salt may also be present insaturated salt muds to increase density or to act as a bridging agentover permeable zones. Starch and starch derivatives for fluid-losscontrol and Xanthan gums for hole-cleaning are among the few highlyeffective additives for salt-water muds.

Sea-water mud is a water based mud designed for offshore drilling whosemake-up water is taken from the ocean. Sea-water has relatively lowsalinity, containing about 3 to 4% by weight of NaCl, but has a highhardness because of Mg⁺² and Ca⁺² ions. Hardness is removed from seawater by adding NaOH, which precipitates Mg⁺² as Mg(OH)₂ and by addingNa₂CO₃, which removes Ca⁺² as CaCO₃. Mud additives are the same as thoseused in freshwater muds: bentonite clay, lignosulfonate, lignite,carboxymethylcellulose or polyanionic cellulose and caustic soda. XCpolymer may also be used in place of bentonite.

Silicate-mud is a type of shale-inhibitive water mud that containssodium or potassium silicate as the inhibitive component. High pH is anecessary characteristic of silicate muds to control the amount and typeof polysilicates that are formed. Mud pH is controlled by addition ofNaOH (or KOH) and the appropriate silicate solution. Silicate anions andcolloidal silica gel combine to stabilize the wellbore by sealingmicrofractures, forming a silica layer on shales and possibly acting asan osmotic membrane, which can produce in-gauge holes throughtroublesome shale sections that otherwise might require an oil mud.

Lime-mud is a type of water based mud that is saturated with lime,Ca(OH)₂, and has excess, undissolved lime solids maintained in reserve.Fluid-loss additives include starch, HP-starch, carboxymethylcellulose(CMC) or polyanionic cellulose (PAC).

Apart from cooling and lubrification of the drilling bit, evacuation ofthe cuttings to the surface and control of the formation pressure,drillings muds must ensure to minimize invasion into permeable zones.

The drilling fluid creates a filter cake that imparts low permeabilityto the face of the permeable formation. The ideal filter cake comprisesa relatively thin and hard layer as opposed to thick viscous coating.Pressure in the bore hole exceeds the pressure in the permeableformation and thereby creates the filter cake which further results inliquid from the drilling fluid moving into the permeable formation. Thisleaves a layer of the filter cake on the face of the hole. Liquidpermeating this filter cake and the formation is called filtrate. As thethickness of the filter cake increases, the volume of fluid loss alsoincreases. The thinner the filter cake, the lower the fluid loss. Athick wall cake will lead not only to high fluid loss, but also to areduction in the diameter of the well bore.

The function of the fluid loss control agents is to delay, prevent or atleast limit as far as possible fluid losses that may be sustained by thedrilling fluids during the drilling operation. However, most of fluidloss control agents used in water based muds such as polyanioniccellulose, carboxymethylcellulose, starch, etc. also give rheology tothe mud. This is therefore limiting the level of fluid loss controlagent that can be used. In this context, an additive that would onlycontrol the fluid loss properties would allow more flexibility in termof level usage. This can become really critical when severe drillingconditions require an ultra-low permeability barrier between the wellbore and the formation.

Additionally, the compounds added to the mud must withstand the hightemperature/high pressure (HTHP) in the wells. Materials that aredescribed in the art as HTHP fluid loss control aids actually have poorstability under these extreme conditions of temperature and pressure.

For high salinity and high temperature conditions,acrylamido-methyl-propane sulfonate polymers (AMPS polymers) have beendeveloped. However, these polymers are showing limited performance above120° C.

SUMMARY OF THE INVENTION

The present invention relates to an improved fluid loss reducer for usein water-based drilling mud. It is a principal object of the presentinvention to provide an improved fluid loss reducer that can be used athigh levels and under conditions of high temperature and high pressurein water-based muds.

This invention is based upon the unexpected discovery that an oilsoluble polymer in a form of a gel can be used as fluid loss additivefor drilling water-based mud compositions. By utilizing such an oilsoluble polymer in drilling water-based muds, greatly improved hightemperature stability and improved shear resistance can be obtained.

The present invention more specifically discloses a water-based drillingmud comprising an aqueous phase wherein the aqueous phase contains anoil soluble polymer in a form of a gel as fluid loss reducer. Thewater-based mud drilling fluid according to the instant inventioncomprises water or salt water, viscosifiers, fluid loss control agents,weighting agents and an oily phase, and other conventional additivesselected from the group consisting of emulsifiers, lubricants, corrosioninhibitors, salts and pH control agents.

The present invention more specifically discloses a water-based drillingfluid mud comprising:

-   -   (1) from 50 to 90% of the aqueous phase,    -   (2) from 0.01 to 0.5% of pH controllers,    -   (3) from 0.1 to 5% of viscosifiers,    -   (4) from 0.01 to 30% of salts,    -   (5) from 0.1 to 3% of emulsifiers,    -   (6) from 4 to 60% of weighting agents,    -   (7) from 0 to 15% of clays, and    -   (8) from 0.1 to 20% of oil soluble polymer in form of a gel as        fluid loss reducer, said percentages being based on the weight        of the mud.

The subject invention further reveals a process for preparing an oilsoluble polymer fluid loss control agent comprising the steps ofdissolving at least one polymer in a hydrocarbon oil to form a clearsolution or a gel, adding an emulsifier to the solution or the gel, andkeeping the mixture under conditions of agitation until a clear creamymixture is obtained.

The present invention also discloses a method of lubricating a drillingbit during the drilling of a well which comprises circulating awater-based drilling mud in the vicinity of the drilling bit, whereinthe water-based drilling mud is comprised of an aqueous phase, andwherein the aqueous phase contains an oil soluble polymer in the form ofa gel as a fluid loss reducer.

DETAILED DESCRIPTION OF THE INVENTION

The water-based drilling fluid mud of the present invention is comprisedof an aqueous phase, wherein the aqueous phase contains an oil solublepolymer in the form of a gel as a fluid loss reducer. The water-baseddrilling fluid mud will typically be comprised of the aqueous phase, apH controller, a viscosifier, a salt, an emulsifier, a weighting agent,clay, and an oil soluble polymer in the form of a gel as a fluid lossreducer. This mud composition will normally contain 50% to 90% of theaqueous phase, 0.01% to 0.5% pH controllers, 0.1% to 5% viscosifiers,0.01% to 30% salts, 0.1% to 3% emulsifiers, 4% to 60% weighting agents,0% to 15% clays, and 0.1% to 20% of oil soluble polymers in the form ofa gel as a fluid loss reducer; based upon the weight of the mud.

The water-based drilling fluid mud will more typically contain 55% to70% of the aqueous phase, 0.1% to 0.3% pH controllers, 0.4% to 2%viscosifiers, 5% to 15% salts, 0.5% to 2% emulsifiers, 10% to 25%weighting agents, 5% to 10% clays, and 0.5% to 2.5% of oil solublepolymers in the form of a gel as a fluid loss reducer; based upon theweight of the mud.

The oil soluble polymer is highly efficient in small proportions as afluid loss reducer. It may be incorporated in an amount of 0.1 to 10%,most preferably in an amount of 0.5 to 2.5% based on the weight of themud.

The suitable polymers for preparing the emulsion used according to theinvention are organo-soluble polymers and may be selected from the groupconstituted by the linear polymers, the grafted polymers, the branchedpolymers and the cross-linked polymers.

A wide variety of polymers or copolymers can be utilized in thisinvention. These polymers may be prepared by polymerization using bulk,emulsion, suspension, solution (anionic, cationic, radical, controlledradical), and condensation polymerization technique. Batch,semi-continuous or continuous polymerization processes are suitable forutilization in the synthesis of the polymer.

The monomers that can be employed are typically selected from the groupconsisting of styrene, substituted styrene, alkyl acrylate, substitutedalkyl acrylate, alkyl methacrylate, substituted alkyl methacrylate,acrylonitrile, methacrylonitrile, acrylamide, methacrylamide,N-alkylacrylamide, N-alkylmethacrylamide, isoprene, butadiene, ethylene,vinyl acetate, vinyl ester of versatic acids containing from 9 to 19carbon atoms (C₉ to C₁₉), and combinations thereof. Functionalizedversions of these monomers may also be used. Some representativeexamples of the selective monomers, which can be used, include styrene,alpha-methylstyrene, para-methylstyrene, para-tertbutylstyrene,vinyltoluene, (M)ethyl(Me)acrylate, 2-ethylhexyl(Me)-acrylate,butyl(Me)acrylate, cyclohexyl(Me)acrylate, isobornyl(Me)acrylate,isobutyl(Me)acrylate,p-terbutyl-cyclohexyl(Me)acrylate, butadiene,isoprene, ethylene, veova, vinyl acetate, acid(Me)acrylic,hydroxy-ethyl(Me)acrylate, glycidyl methacrylate, sodiumbenzenesulfonate and combinations thereof.

The typical but non-limiting examples of suitable polymers that areuseful in this invention are commercially available from a wide varietyof sources. For instance, Eliokem sells styrene-acrylate orstyrene-butadiene copolymers under the trade name PLIOLITE®, such asPLIOLITE® DF01 polymer, PLIOLITE® DF02 polymer, PLIOLITE® DF03 polymer,PLIOLITE® VTAC-H polymer, PLIOLITE® VT polymer, and PLIOLITE Ultra 200®polymer, substituted styrene-acrylate copolymers under the trade namePLIOWAY®; Rohm sells acrylate resins under the trade name PLEX®,Goodyear Chemicals sells styrene-butadiene rubber under the trade namePLIOFLEX®; Kraton S.A. sells block copolymers styrene-butadiene andtheir hydrogenated version under the trade name KRATON®.

A wide variety of crosslinking agents can be utilized in carrying outthe polymerization to produce these polymers. Some representativeexamples of crosslinking agents which can be utilized includedifunctional acrylates, difunctional methacrylates, trifunctionalacrylates, trifunctional methacrylates, allyl maleate anddivinylbenzene.

Various other components are added to provide the desired properties forthe drilling mud. Various polymers are used to control the mud rheologyand therefore to keep the rock cuttings in suspension as they move up tothe borehole to the surface. An alkaline substance, such as causticsoda, provides alkalinity to the system. A weighting agent, such asbarite, is also used. The water-mud may be a potassium mud, a salt watermud, a sea water mud, or a lime mud.

The present invention further reveals a process for preparing the oilsoluble polymer fluid loss control agent. The oil soluble polymer fluidloss control agent may be prepared by dissolving polymers in ahydrocarbon oil to form a clear solution or a gel, adding an emulsifierand keeping the mixture under stirring until a clear creamy mixture isobtained; optionally an emulsion can be prepared by adding water underhigh shear stirring.

The oil soluble polymer thus obtained is added to a water-based mudprepared by conventional methods, either in replacement of theconventional fluid loss reducers, or in addition to said conventionalfluid loss reducers.

The organic liquids which may be utilized in the instant invention areselected with relation to the polymer solubility. The hydrocarbon oil isselected from the group comprising of aromatic hydrocarbon, chlorinatedaliphatic hydrocarbons, aliphatic hydrocarbons, cyclic aliphatic ethers,aliphatic ethers, or organic aliphatic esters and mixture thereof.Preferably hydrocarbon oils are selected from the groups of synthetichydrocarbons and organic aliphatic ester, most preferably from the groupof well fluids (base oil) useful in the rotary drilling process. Theproperties of the base oil can vary however, so that it is usuallynecessary to perform solubility test to determine the appropriate amountof oil to prepare the emulsion.

The level of emulsifier added to the polymer solution is typically 1 to30%, preferably from 3 to 20%, most preferably from 5 to 15% based onthe weight of the polymer solution/emulsifier mixture. The preferredemulsifiers for use in the instant invention include ionic and non ionicderivatives and mixtures thereof. Specific examples of preferredemulsifiers that are useful in the instant invention include alkylsulfate, alkyl benzene sulfonate, alkyl ethersulfates, sulfonated oleicacid, alkylphenol ethersulfates, sulfosuccinates, phosphoric ester,fatty acid amides, fatty acid amines, fatty alcohol polyglycolethers,modified fatty alcohol polyglycolethers, alkyl polyglycosides, modifiedalkyl polyglycosides, alkylphenol polyglycolethers, fatty acidpolyglycolethers, sorbitan fatty acid esters. More preferably, theemulsifier is chosen in the group comprising alkyl ethersulfates andfatty acid amides and their derivatives.

Optionally, water can be added to the polymer solution/emulsifiermixture. The level of water is typically 0 to 60%, preferably from 10 to50%, most preferably from 20 to 40% based on the weight of the polymeremulsion.

The level of polymers in the polymer solution is typically about 1 toabout 40%, preferably from 5 to 35%, most preferably from 15 to 25%based on the weight of the polymer solution. It is important for thepolymer backbones to be soluble in the organic liquid.

The present invention further reveals the use of an oil soluble polymerin a form of a gel as a fluid loss reducer in a water-based mud. Theinstant invention further reveals a method of lubricating a drill pipewhen drilling well, said method comprising circulating a water-baseddrilling fluid containing an oil soluble polymer in a form of a gel as afluid loss reducer.

The oil soluble polymers in a form of a gel, according to the instantinvention, allow a good thermal stability and avoid any rheologicalcontribution. They may be used at high temperature and high pressureconditions. Moreover, they improve the quality of the filter cake andalso improve the water-based muds lubricity performance.

This invention is illustrated by the following examples that are merelyfor the purpose of illustration and are not to be regarded as limitingthe scope of the invention or the manner in which it can be practiced.Unless specifically indicated otherwise, parts and percentages are givenby weight.

EXAMPLE 1 Preparation of the Oil Emulsion and of the Polymer in a Formof a Gel

The preparation of an oil emulsion is as follows:

First, 100 grams of RADIAGREEN® base oil is mixed with 33.3 grams ofDISPONIL FES® emulsifier and kept under stirring for 5 minutes. Then,36.7 grams of water are slowly incorporated to the oil/emulsifiermixture which is stirred under high shear for 20 minutes. If needed,defoamer is added to the preparation. The particle size of the resultingemulsion is then measured.

The polymers in a form of a gel are prepared using the same recipe: 100grams of base oil is replaced by 100 grams of 80/20 polymer gel which isprepared by the dissolution of 20 grams of polymers in 80 grams of baseoil. The polymers used were either linear, grafted, branched orcross-linked.

EXAMPLE 2 Preparation of the Water-based Muds

The base muds were prepared by utilizing a conventional laboratorymethod. The muds were then placed in aging cells, pressurized at 6.9×10⁵Pa (100 psi), and heated for 4 hours at 95° C. (203° F.) and/or 16 hoursat 95° C.(203° F.) and/or 16 hours at 130° C. (266° F.). The aging cellswere cooled down to room temperature, depressurized and then therheology of the mud was measured on a Fann Model 35 viscometer at 50° C.(122° F.). Static filtration measurement was performed with standard APIfiltration cell at 6.9×10⁵ Pa (100 psi) and 25° C.(77° F.).

In order to test the emulsion property at high pressure and hightemperature, static filtration measurements at 34.5×10⁵ Pa (500 psi) and95° C.(203° F.) were performed. The results, obtained with thewater-based mud containing the polymeric emulsion, are directly comparedwith the water-based mud (control) and the water-based mud containingthe oil emulsion.

The different mud formulations used are shown in Table I.

TABLE I Formulation 1 Formula- Formula- (water-based mud) tion 2 tion 3INGREDIENTS (g) (g) (g) Freshwater 276.5 265.2 265.2 KCl 45 45 45Caustic Soda 0.3 0.3 0.3 Soda Ash 0.3 0.3 0.3 Polyanionic 0.75 0.75 —Cellulose 1 Polyanionic 3.0 3.0 — Cellulose 2 Xanthan gum 0.75 0.75 1.25OCMA clay 35 35 35 Barite 58.31 58.31 58.31 Emulsion — 31.9 31.9

Formulation 1 (base-mud) is reference based mud comprising ioniccellulose as fluid loss reducer.

Formulation 2 is a water-based mud with the same composition asformulation 1, but also containing an oil soluble polymer according tothe instant invention as fluid loss reducer (“on top” formulation). Forthe preparation of formulation 2a, the emulsion does not contain anypolymer. For the preparation of formulation 2b, a linear polymer isused. For the preparation of formulation 2c, a cross-linked polymer isused.

In formulation 3, polyanionic cellulose is replaced by the oil solublepolymer according to the instant invention. A cross-linked polymer isused.

The results obtained with the non cross-linked and the cross-linkedpolymers emulsion over the temperature range of 25° C.-95° C.(77°C.-200° F.) are directly compared with the water-based mud (control).

As shown in Table II, the polymeric emulsion gives significantly betterresults in term of static filtration than the reference formulationwithout having any impact on mud rheology.

TABLE II Rheology Filtration Fann 35 SA viscometer Fann HTHP filter 600300 200 200 6 3 Plastic Yield point press rpm rpm rpm rpm rpm rpmviscosity (lb/100 ft² = Room t° C. 95° C. Muds formulations (lb/100 ft²= 0.479 Pa) (cps = 10⁻⁴ Pa.S) 0.479 Pa) ml ml Formulation 1 78 52 43 319 7 26 26 4.0 13.6 76 51 43 30 8 6 25 26 4.8 14.0 Formulation 2a: 79 5442 28 7 5 25 29 2.0 10.0 Oil emulsion (on top) Formulation 2b: VTACH* 7348 37 25 6 4 25 23 1.6 8.0 Linear polymer XPR036*¹ 73 53 45 33 13 11 2033 2.4 8.8 (on top) Pliolite VT* 78 56 46 33 8 6 22 34 — 8.4 Kraton G*86 60 49 34 8 6 26 34 2.0 8.4 Formulation 2c: Pliolite DF01* 75 51 40 287 5 24 27 1.6 6.4 Cross-linked DF01-NaSS* 86 59 48 32 7 6 27 32 1.6 6.4polymer Pliolite DF02* 90 62 50 34 8 6 28 34 — 5.6 (on top) PlioliteDF03* 89 61 48 33 7 5 28 33 — 6.0 CPR 7676* 88 59 47 32 7 5 29 30 1.65.6 CPR 7755* 88 60 49 33 8 6 28 32 1.6 6.0 Formulation 3: PlioliteDF01* 33 18 14 11 4 3 5 3 4.8 — Cross-linked polymer (replacement) ¹Aresubstituted styrene acrylate copolymers.

Filtration value in ml is significantly reduced when polymer such asVTACH® U200 polymer, XPR036 polymer, PLIOLITE® VT polymer, or Kraton® Gpolymer was added in re-emulsified form in the standard water-baseddrilling fluid formulation. This value is further reduced when the abovepolymer is replaced by a cross-linked polymer such as PLIOLITE® DF01,DF02, DF03, CPR7676, and CPR7755 polymers. Replacement in water-basedmud of fluid loss reducers by the polymer emulsion leads to equivalentlevel of filtration versus water-based mud.

As can be seen in Table III, filtration value in ml is significantlyreduced after high temperature aging when cross-linked polymer such asPLIOLITE® DF01 was used with an emulsifier package (DISPONIL® FESemulsifier and KLEEMUL® emulsifier) in the standard water-based drillingfluid formulation.

While certain representative embodiments and details have been shown forthe purpose of illustrating the subject invention, it will be apparentto those skilled in this art that various changes and modifications canbe made therein without departing from the scope of the subjectinvention.

1. A method of lubricating a drilling bit during the drilling of a wellwhich comprises circulating a water-based drilling mud in the vicinityof the drilling bit wherein the water-based drilling mud is comprised of(1) from 50 to 90% of the aqueous phase, (2) from 0.01 to 0.5% of pHcontrol agent, (3) from 0.1 to 5% of viscosifiers, (4) from 0.01 to 30%of salts, (5) from 0.1 to 3% of emulsifiers, (6) from 4 to 60% ofweighting agents, (7) from 0 to 15% of clays, and (8) from 0.1 to 20% ofan oil soluble polymer in form of a gel as a fluid loss reducer, saidpercentages being based on the weight of the water-based drilling mud,wherein the oil soluble polymer in the form of a gel is prepared by aprocess comprising the steps of dissolving at least one polymer in ahydrocarbon oil to form a clear solution or a gel, adding an emulsifierto the solution or the gel, and keeping the mixture under conditions ofagitation until a clear creamy mixture is obtained; wherein said oilsoluble polymer fluid loss control agent contains up to 50 percent waterbased on the weight of the control agent; wherein the emulsifier ispresent at a level which is within the range of 3 percent to 30 percentbased upon the weight of the control agent; and wherein the polymer isprepared from monomers selected from the group consisting of styrene,substituted styrene, alkyl acrylate, substituted alkyl acrylate, alkylmethacrylate, substituted alkyl methacrylate, acrylonitrile,methacrylonitrile, acrylamide, methacrylamide, N-alkylacrylamide,N-alkylmethacrylamide, isoprene, butadiene, ethylene, vinyl acetate, andvinyl esters of versatic acids containing from 9 to 19 carbon atoms. 2.A method as specified in claim 1 wherein the water-based drilling mud isutilized in conjunction with conventional fluid loss reducers.
 3. Amethod as specified in claim 1 wherein the method is conducted in theabsence of conventional fluid loss reducers.
 4. A method as specified inclaim 1 wherein the water-based drilling mud is further comprised of atleast one lubricant.
 5. A method as specified in claim 1 wherein thewater-based drilling mud is further comprised of at least one corrosioninhibitor.
 6. A method as specified in claim 1 wherein the water-baseddrilling mud is further comprised of at least one fluid loss controlagent.
 7. A method as specified in claim 1 wherein the water-baseddrilling mud is comprised of: (1) from 55 to 70% of the aqueous phase,(2) from 0.1 to 0.3% of pH controllers, (3) from 0.4 to 2% ofviscosifiers, (4) from 0.5 to 15% of salts, (5) from 0.5 to 2% ofemulsifiers, (6) from 10 to 25% of weighting agents, (7) from 5 to 10%of clays, and (8) from 0.5 to 2.5% of oil soluble polymer in form of agel as fluid loss reducer, said percentages being based on the weight ofthe mud.
 8. A method as specified in claim 1 wherein the oil solublepolymer is incorporated into the water-based drilling mud in an amountwhich is within the range of 0.1% to 10%, based on the weight of themud.
 9. A method as specified in claim 1 wherein the oil soluble polymeris prepared from monomers selected from the group consisting of styrene,substituted styrene, alkyl acrylate, substituted alkyl acrylate, alkylmethacrylate, substituted alkyl methacrylate, acrylonitrile,methacrylonitrile, acrylamide, methacrylamide, N-alkylacrylamide,N-alkylmethacrylamide, isoprene, butadiene, ethylene, vinyl acetate, andvinyl esters of versatic acids containing from 9 to 19 carbon atoms. 10.A method as specified in claim 1 wherein the hydrocarbon oil is selectedfrom the group consisting of aromatic hydrocarbons, chlorinatedaliphatic hydrocarbons, aliphatic hydrocarbons, cyclic aliphatic ethers,aliphatic ethers and organic aliphatic esters.
 11. A method as specifiedin claim 1 wherein the emulsifier is a member selected from the groupconsisting of alkyl sulfates, alkyl benzene sulfonates, alkylethersulfates, sulfonated oleic acid, alkylphenol ethersulfates,sulfosuccinates, phosphoric ester, fatty acid amides, fatty acid amines,fatty alcohol polyglycolethers, modified fatty alcohol polyglycolethers,alkyl polyglycosides, modified alkyl polyglycosides, alkylphenolpolyglycolethers, fatty acid polyglycolethers, and sorbitan fatty acidesters.
 12. A method as specified in claim 1 wherein the emulsifier isselected from the group consisting of alkyl ether sulfates and fattyacid amides.
 13. A method as specified in claim 1 wherein the weightingagent is barite.
 14. A method as specified in claim 6 wherein the fluidloss control agent is selected from the group consisting ofcarboxymethylcellulose, polyanionic cellulose, and starch.
 15. A methodas specified in claim 1 wherein the clay is present at a level which iswithin the range of 5 weight percent to 10 weight percent, based on theweight of the water-based drilling mud.
 16. A method as specified inclaim 7 wherein the fluid loss control agent is starch.
 17. A method asspecified in claim 7 wherein the fluid loss control agent iscarboxymethylcellulose.